In our tech group that was one of the things we talked about, that we enjoyed being able to physically write on printed materials, especially for learning purposes. Dr. K shared with us something called Crocdoc (http://personal.crocodoc.com/). It allows you to mark on PDFs the way that you could write on a printed text. Theoretically you could copy and paste text from a web page and make a PDF to write on with Crocdoc if the original text was not in PDF. Neat huh?